VIBRATION SOURCES

تاريخ التسجيل : 22/11/2009

4.1.
What Is Diagnosis?

 From
a general perspective, including both the medical and technical case, diagnosis
can be explained as follows. For a process there are observed variables or
behaviors for which there is knowledge of what is expected or normal. The task
of diagnosis is to, from the observations and the knowledge, generate a diagnosis,
i.e. to decide whether there is a fault or not and also to identify the fault.
The following picture shows the information flow including a process and a
diagnosis system.

4.2. Vibration Sources


All machinery with moving parts generates mechanical forces during
normal operation. As the mechanical condition of the machine changes because of
wear, changes in the operating environment, load variations, and so on, so do
these forces. Understanding machinery dynamics and how forces create unique
vibration frequency components is the key to understanding vibration sources.




Vibration does not just happen. There is a physical cause, referred to
as a forcing function, and each component of a vibration signature has its
own forcing function. The components that make up a signature are reflected as
discrete peaks in the FFT or frequency-domain plot.



The vibration profile that results from motion is the result of a force
imbalance. By definition, balance occurs in moving systems when all forces
generated by, and acting on, the machine are in a state of equilibrium. In
real-world applications, however, there is always some level of imbalance, and
all machines vibrate to some extent. This section discusses the more common
sources of vibration for rotating machinery, as well as for machinery
undergoing reciprocating and/or linear motion.



4.2.1.
Unbalance

<p class="MsoNormal" style="text-align:justify;mso-pagination:widow-orphan lines-together;
tab-stops:right 18.0pt;mso-layout-grid-align:none;text-autospace:none;
direction:ltr;unicode-bidi:embed" align="left"> Vibration due to unbalance of a rotor is
probably the most common machinery defect. It is luckily also very easy to
detect and rectify. The International Standards Organisation (ISO) define
unbalance as: That condition, which exists in a rotor when
vibratory, force or motion is imparted toits bearings as a result of
centrifugal forces. It may also be defined as: The uneven
distribution of mass about a rotor’s rotating centerline.
There are two new terminologies used: one is rotating centerline
and the other is geometric
centerline. The rotating centerline
is defined as the axis about which the rotor would rotate if not
constrained by its bearings (also called the principle inertia
axis or PIA). The geometric centerline (GCL)
is the physical centerline of the rotor. When the two centerlines are
coincident, then the rotor will be in a state of balance. When they are apart,
the rotor will be unbalanced. There are three types of unbalance that
can be encountered on machines, and these are:

1.
Static unbalance (PIA and GCL are
parallel) figure (4-2)

2.
Couple unbalance (PIA and GCL intersect
in the center) figure (4-3)

3.
Dynamic unbalance (PIA and GCL do not touch or coincide).



For all types of unbalance, the FFT
spectrum will show a predominant 1× rpm
frequency of vibration. Vibration amplitude at the 1× rpm frequency will vary proportional to the square of the rotational
speed.

4.2.2.
Misalignment

 Misalignment, just like unbalance, is a
major cause of machinery vibration. Some machines have been incorporated with
self-aligning bearings and flexible couplings that can take quite a bit of
misalignment. However, despite these, it is not uncommon to come across high
vibrations due to misalignment.

<p class="MsoNormal" style="text-align:justify;text-indent:18.0pt;mso-pagination:
widow-orphan lines-together;mso-layout-grid-align:none;text-autospace:none;
direction:ltr;unicode-bidi:embed" align="left">There are basically two types of
misalignment:

1.
Angular misalignment: the shaft centerline
of the two shafts meets at angle with each other

2.
Parallel misalignment: the shaft centerline
of the two machines is parallel to each other and have an offset.

Misalignment primarily subjects the
driver and driven machine shafts to axial vibrations at the 1× rpm frequency. The figure is an exaggerated and simplistic single-pin
representation, but a pure angular misalignment on a machine is rare. Thus,
misalignment is rarely seen just as 1× rpm
peak. Typically, there will be high axial vibration with both 1× and 2× rpm. However, it is not unusual for 1×, 2× or 3× to
dominate. These symptoms may also indicate coupling problems

4.2.3. Eccentric rotor

Eccentricity occurs when the center of
rotation is at an offset from the geometric centerline of a sheave, gear,
bearing, motor armature or any other rotor. The maximum amplitude occurs at 1× rpm of the eccentric component in a direction through the centers of the
two rotors. Here the amplitude varies with the load even at constant speeds
In a normal unbalance defect, when the
pickup is moved from the vertical to the horizontal direction, a phase shift of
90° will be observed. However in eccentricity, the phase readings differ by 0
or 180° (each indicates straight-line motion) when measured in the horizontal
and vertical directions. Attempts to balance an eccentric rotor often result in
reducing the vibration in one direction, but increasing it in the other radial
direction (depending on the severity of the eccentricity)

4.2.4.
Bent shaft

 When a bent shaft is encountered, the
vibrations in the radial as well as in the axial direction will be high. Axial
vibrations may be higher than the radial vibrations. The FFT will normally have
1× and 2× components.
If the:

1•
Amplitude of 1× rpm
is dominant then the bend is near the shaft center figure (4-14)

2•
Amplitude of 2× rpm
is dominant then the bend is near the shaft end.

4.2.5.
Rolling element bearings



A
rolling element bearing comprises of inner and outer races, a cage and rolling
elements. Defects can occur in any of the parts of the bearing and will cause
high-frequency vibrations. In fact, the severity of the wear keeps changing the
vibration pattern. In most cases, it is possible to identify the component of
the bearing that is defective due to the specific vibration frequencies that
are excited. Raceways and rolling element defects are easily detected. However,
the same cannot be said for the defects that crop up in bearing cages. Though
there are many techniques available to detect where defects are occurring,
there are no established techniques to predict when the bearing defect will
turn into a functional failure. In an earlier topic dealing with
enveloping/demodulation, we saw how bearing defects generate both the bearing
defect frequency and the ringing random vibrations that are the resonant
frequencies of the bearing components. Bearing defect frequencies are not
integrally harmonic to running speed. However, the following formulas are used
to determine bearing defect frequencies. There is also a bearing database
available in the form of commercial software that readily provides the values
upon entering the requisite bearing number.



4.2.6.
Gearing defects

A
gearbox is a piece of rotating equipment that can cause the normal
low-frequency harmonics in the vibration spectrum, but also show a lot of
activity in the high frequency region due to gear teeth and bearing impacts.
The spectrum of any gearbox shows the 1× and 2× rpm, along with the gear mesh frequency (GMF).
The GMF is calculated by the product of the number of teeth of a pinion or a
gear, and its respective running speed

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